1. Field of the Invention
The present invention relates to a pattern inspection apparatus which inspects defects in a pattern, and relates in particular to a pattern inspection apparatus which inspects defects included in the pattern of a mask, a wafer, a liquid crystal substrate and the like used when a semiconductor element or a liquid crystal display (LCD) is manufactured.
2. Description of the Related Art
Recently, a pattern inspection apparatus has been developed which inspects a pattern by comparing design pattern data and detected pattern data on a mask used for production of a large-scale integration, and in this pattern inspection apparatus, a reflection optics is mounted in addition to a transmission optics so as to improve detection sensitivity (refer to “Performance of cell-shift defect inspection technique”, Photo mask and X-Ray Mask Technology IV, Vol. 3096 (1997), pp 404-414). In this apparatus, a difference is made between a wavelength used in a transmitted-light-based inspection and a wavelength used in a reflective-light-based inspection so as to separate the wavelengths by a filter in an optical system to be configured, and each light is put into a transmitted/reflected light detection sensor.
However, it is necessary to shorten the inspection wavelengths in order to enhance detection sensitivity, and further in order to perform an inspection conforming to the wavelength used in lithography. As the shortened inspection wavelengths complicate designing of an optical lens, it is especially difficult to design a lens reducing aberration in two wavelengths. This has posed a problem that it is difficult to adopt the optical system with the changed wavelengths of the transmitted/reflected lights in such an inspection apparatus that detects a defect size of 100 nm. Therefore, a method is needed to obtain transmission and reflection images by use of a single wavelength.
When both the transmitted light and reflected light are utilized for observation, in general, the same place was coaxially irradiated to acquire an observation image (e.g., refer to U.S. Pat. No. 5,572,598, U.S. Pat. No. 5,563,702). These methods adopt a beam scan technique and show an adequate consideration to acquisition of the transmission image and reflection image. However, when the same area is observed, the image needs to be optically separated in some way. A light amount loss is relatively small when the two wavelengths are separated by the filter as has conventionally been done, but the light amount loss is larger in the case of the single wavelength because the light amount to be obtained is half in a method that separates by a half mirror or the like.
Furthermore, a laser is often used as a short-wavelength light source, and a polarizing splitting method is used to separate a light of the laser. However, it has been pointed out in connection with the is polarization split that complete separation of the transmission and reflection is difficult and a mutual interference occurs, that the light amount decreases because polarization efficiency is decreased by a short-wavelength polarizing beam splitter, and that when the polarization split (such as the polarizing beam splitter) is used, a polarizing plate such as a λ/2 or λ/4 plate has to be inserted into a part of the optical system, thereby losing some optical amount in this part. Moreover, because an objective lens, which is most important among optical components, needs to be placed at a position facing a substrate surface, a configuration of the optical system is significantly complicated and expensive.
On the other hand, an optical system inspection method and apparatus have been proposed which separate transmitted light irradiation and reflected light irradiation without applying a beam to the same point so as to irradiate the beam within the same field of view of an observation optics (refer to Published Japanese translations of PCT international publication No. 2002-501194). Since this method in which the observation optics is separated using only one objective lens that is the most important optical component, it can make up for weak points of U.S. Pat. No. 5,572,598, U.S. Pat. No. 5,563,702 mentioned above.
However, this method has a great loss of light amount in ways of irradiating an incident beam, separating the field of view, and leading the transmitted light/reflected light to the detection sensor. It especially has a disadvantage in that the light amount loss is not considered in a concept of splitting the transmitted light/reflected light from a reflection beam introductory part to the detection sensor. Moreover, an optical system is not introduced which can change the magnification independently in each part from the reflection beam introductory part to the transmitted light/reflected light detection sensor. Even if the detection sensors are laid out in close proximity to conduct a detection, a design that places the sensors in close proximity is realistically difficult in view of an optical magnification required for the inspection and a physical size of the sensors. In addition, a method has also been proposed for splitting the beam with a beam splitter in this part, but such a problem still remains that the light amount loss is not considered as described above.
From now on, when the inspection wavelength is shortened to 200 nm or lower, deterioration of light amount is a problem that needs the most attention. Further, in the abovementioned document, a simultaneous inspection with the transmitted light/reflected light is not considered in view of a difference of the light amounts to be obtained in the transmitted light/reflected light, leaving enormous difficulties in actual operation.
As described above, the conventional pattern inspection apparatus has a concept of utilizing both the transmitted light and reflected light, but it does not effectively use a light source power needed for the inspection apparatus. Moreover, the light amount loss is great in splitting the transmitted light and the reflected light within an inspection field, in a situation where the wavelength is being shortened, thus posing a problem of sensitivity decrease.
Therefore, is has been desired to realize a pattern inspection apparatus which can inspect pattern defects on a substrate by use of both the transmitted light and reflected light, which reduces the light amount loss associated with the splitting of the transmitted light and reflected light within the inspection field of a short-wavelength optical system, and which can conduct an inspection with satisfactory sensitivity.